When we traveled from Zurich to Siena, we took the train to Stresa, stayed two nights, and then took the train to Florence & the bus to Siena. Beginning from Geneva would be similar. We had been to Siena, previously, so we knew that the bus will leave them off in the city close to the hotel where we stayed.
Stresa is a nice spot to stay - beautiful historical elegance, plus the small islands to explore - Isola Bella’s villa & gardens, Isola Pescatori is a tiny fishing village with a few restaurants. (I’ve returned a few times to spend more days in this area,)
Otherwise, they could stop at Milan as their midpoint place.
Traveling by train is very easy. We’ve been in Europe a dozen times and have never rented a car,